1. Technical Field
The present invention relates to a step-down DC-to-DC converter.
2. Related Art
FIG. 9 is a view showing a configuration of a step-down DC-to-DC converter in the related art. The step-down DC-to-DC converter in the related art is formed of a chopper switching device Q1 and a synchronous rectification switching device Q2 sequentially connected in series to an input power supply E, a reactor L1 connected sequentially in series between a connecting point of the switching devices Q1 and Q2 and a ground point, an output smoothing capacitor C1, and a drive circuit DR controlling the chopper switching device Q1 to switch ON and OFF. By allowing the drive circuit DR to output a drive signal to the chopper switching device Q1 so that an ON state of a predetermined time T1 and an OFF state of a predetermined time T2 are repeated alternately, an input voltage from the input power supply E is stepped down to a predetermined output voltage and supplied to a load L connected in parallel with the output smoothing capacitor C1.
FIG. 10 shows operation waveform charts of the step-down DC-to-DC converter in the related art. The abscissas are used for a time t and the ordinates from top to bottom are used for a positive terminal voltage of the switching device Q1, a negative terminal voltage of the switching device Q1, and a voltage across the switching device Q1, Vsw. Herein, when the switching device Q1 switches OFF from ON, a surge voltage Vsg expressed as L×di/dt is generated due to the presence of wire-induced parasitic inductance between the switching device Q1 and the input power supply E. Accordingly, an input voltage, Vin+Vsg, is applied to the positive terminal of the switching device Q1. Hence, it is necessary to design the switching device Q1 in considerations of Vin+Vsg. To this end, it is necessary to use a switching device having high breakdown voltage and slow down a switching speed. In short, it is necessary to increase 1/(di/dt).